The following relates to the nuclear fuel arts, nuclear reactor arts, nuclear power generation arts, and related arts.
A nuclear reactor core is typically constructed as an array of fuel assemblies (FA's) in which each FA is vertically coextensive with the height of the reactor core and the array of FA's spans the lateral dimensions of the reactor core. Each FA comprises an array of vertically oriented fuel rods held together by a structural skeleton comprising a set of horizontal spacer grids spaced apart along the vertical direction which are welded to guide tubes or other rod-like vertical elements. The upper and lower ends of the FA are capped by upper and lower end fittings (also sometimes called nozzles) connected to the guide tubes by fasteners, welding, or the like.
Conventional spacer grids are constructed by interlocking straps, where each strap is machined (e.g. stamped) from a strip of metal, such as a nickel-chromium alloy (e.g., Inconel™) strip or a zirconium alloy (e.g., Zircaloy™) strip. The intersecting straps define openings, also called cells, through which fuel rods pass. The straps are machined or stamped to define dimples (i.e., “hard” stops, protrusions having high stiffness) and springs (i.e. “soft” stops, protrusions having low stiffness) in each cell to hold the fuel rod passing through the cell. Typically two dimples are formed from the straps forming two adjacent walls in each square cell. One dimple in each pair is located near the top of the grid strap and the other is located near the bottom of the grid strap. The opposite cell walls each contain a single spring which may either be formed from the strap that makes that cell wall, or in the case of a bi-metallic spacer grid, may be an insert made of a different material that is mechanically trapped or restrained by features formed from the strap that make up that cell wall. The springs are located at or near the mid-plane of the spacer grid, and are sized so that an interference condition exists when a fuel rod is inserted into the grid cell. This interference causes the springs to deflect backwards towards the cell walls on which they are located, preloading the fuel rod in two orthogonal directions against the opposing dimple pair and clamping it in position. The axial offset between the plane of action of the springs and the plane of action of the dimples creates restoring moments that cause the local vertical orientation of the fuel rod at the spacer grids to remain relatively fixed should lateral forces be applied to the fuel rod between any two axially adjacent spacer grids.
Adverse handling conditions can produce high accelerations on the fuel rods which in turn may over-deflect the springs. Springs that are deflected past the normal fuel rod location may be plastically deformed and lose grip force on the fuel rod. Springs may also bottom out, allowing the fuel rod to contact the grid.
The following discloses various improvements.